Confused myself about Minolta mounts. Apparently the old MD
mount is around 44.72mm, but the newer autofocus mount is listed
as 44.5, 44.6, and 44.7, depending on where you look. I thought I
had found the AF mount, but the camera involved was an MD mount.

Added a reference to the Sigma SA mount, which resembles the Pentax
K, but with different meter, aperture, and autofocus connections,
and with a smaller register distance.

Clarified the Icarex mount situation. Some later models used
the M42 thread mount. I am assuming that the “bayonet” Icarex
mount is the same as the “breech lock” mentioned in the Markerink
and KineOptics lists.

Altered the Miranda entries. I believe there was only one
mount and that the auxiliary screw mount has a diameter of 44mm,
not 42 or 46. The latter seems to be a filter mount. See this
page and this
one for more info.

Basic Issues with Adapting Lenses

At first glance, adapting a lens to fit a camera seems quite
straightforward, if the two registers relate in the correct way. That
is, the new camera has a register distance less than that
of the camera originally intended for the lens. Unfortunately, several
things can get in the way.

First, lens mounts for movie cameras are generally designed so the
lens extends quite far into the camera behind the flange surface. I
suspect this is done to get a much stiffer, stronger connection
between lens and camera. Lenses for SLR still cameras don't extend
very far into the camera, because space is needed for the mirror to
move.

Second, the diameter of the mount matters: your lens might be too
fat to attach at the correct register.

Third, some lens mounts, notably the Contax
rangefinder, put the focusing mechanism in the camera body; most
adapters or modifications will need to include a means to focus the
lens.

Fourth, some cameras (e.g. Zeiss Contaflex, Kodak Retina IIIc)
don't let you remove the entire lens, but offer interchangeable front
elements to provide different focal lengths. Needless to say, adapting
such half-lenses to another camera has little chance of giving any
useful result. I believe the
interchangeable elements for the Contaflex were all named
“Pro-Tessar”.

Finally, some lens mounts depend on the camera body to set the
aperture; the Contarex and Icarex mounts fall in this category, as
does the obscure mount of the Kiev 10 and 15. Any adapter will need to
provide some mechanism to control aperture. An increasing number of
modern lenses also fall into this category. A few mounts (e.g. Pentax
110 and Canon SD lenses for the Demi C) put the iris in the body, so
the lenses lack it.

Functionality

Back in the 1930s, the only connection between lens and camera body
was structural; the lens mount simply had to hold the lens at a
precise distance from the film. As time went on, however, things got
more complicated. First came automatic stop-down of the lens aperture
for SLRs: the photographer could focus and frame the shot with lens
wide open, and as soon as the shutter release was pressed, the lens
would stop down to an aperture set beforehand. Then, as SLRs acquired
through-the-lens metering systems, the lenses of some cameras had a
mechanism to signal what aperture had been selected. After that came
automatic exposure, and the camera needed to control the lens aperture
(and to know the aperture range of the lens). Finally came automatic
focus: the camera could focus the lens, mechanically or electrically.

These more sophisticated connections, however, are far harder to adapt
between different lens mounts. Once you have built an adapter (or
modified mount) to connect a lens from system A to camera body B, you
still lack the connections to make automatic aperture, exposure,
and focusing work. Typically this means that many functions are lost;
you stop the lens down manually, use manual exposure, etc. This can
run afoul of the electronics of modern SLRs; Canon SLR's, for example,
usually require a chip in the adapter if you want to use the autofocus
system as a guide to focussing manually. Realistically, this
full-manual functionality is all that can be hoped for, given the
diversity of mechanisms and the lack of space to build any sort of
lever arrangement.

These difficulties exist even where such adapters were planned as part
of the mount design; Pentax, for example, offered an adapter to use
M42 lenses on the K bayonet, but none of the automation works. The
Contax/Yashica bayonet is so similar to the Pentax K that apparently a
bit of work with a file or Dremel tool can get one to mount on the
other, but the aperture mechanisms are completely different.

One happy exception is the Rollei/Voigtländer QBM mount used in the
Rolleiflex SL35 and kin; this uses a stop-down pin copied from the M42
mount, so an M42 lens on a Rollei with the factory-supplied adapter
retainss the original aperture function of the lens. Another is the
Sigma SA mount, which copies the electronic protocol of the Canon EF
mount, as well as its register distance. Apparently it is possible to
modify a Canon lens with a Sigma mount, or a Sigma body with a Canon
mounting flange, and have full automation and autofocus. It is also
possible, in principle, to build electronics to convert between lens
protocols for two systems; a company called Conurus offers such a modification for Contax N lenses to mount
on Canon EF or Sigma bodies.

42.1mm according to this
list, 42.00mm according to Wikipedia, 42.13mm according to
a Japanese list that seems to have vanished from the Web. The Canon service manuals of the time specify a “42.14 Dial Gauge”,
whatever that might be, to adjust the flange distance

Measured to outside of outer bayonet; see this
page. Most lenses mount to inner bayonet and have no focusing helical; focus is in camera body.
Lenses mounting to the outer bayonet (usually longer lenses) have focusing capability.

Identical to Exakta, but with different mechanism for automatic diaphragm.

Topcon UV

bayonet

55

Used on leaf-shutter cameras with shutter behind the lens.

Voigtländer Bessamatic

bayonet

44.7

This is a leaf-shutter camera, but designed with the shutter
behind the lens, so the entire lens can be removed and
used elsewhere. The aperture ring is part of the camera, not
part of the lens. Someone will sell you a Nikon F adapter for this mount that includes
an aperture control ring. The Bessamatic mount is so similar to
that of the Kodak Retina Reflex that
apparently it is possible to modify a lens to fit both cameras.

Voigtländer Vitessa T

bayonet

44.7

A variant of the Deckel mount,but
including the aperture control ring in the lens, rather than in the
body. Also used on the Braun Colorette.

This mount predates the M42-mount Yashica SLR’s, which
were introduced in 1962. Judging by the photos of Exakta and M42
adapters found in the
manual, the register is significantly less than the 45.46mm of
the M42 mount. Not to be confused with the shared Contax/Yashica
bayonet mount, which has a register similar to that of the M42
mount.

ZM39

M39×26tpi

45.46

39

An oddity that apparently uses the Leica
thread, but with the same register as M42. Used on Zenit 1, Zenit S, Zenit 3, Zenit
3M, and Kristall.